Gallium nitride cross-gap light emitters based on unipolar-doped tunneling structures

ABSTRACT

Gallium nitride based devices and, more particularly to the generation of holes in gallium nitride based devices lacking p-type doping, and their use in light emitting diodes and lasers, both edge emitting and vertical emitting. By tailoring the intrinsic design, a wide range of wavelengths can be emitted from near-infrared to mid ultraviolet, depending upon the design of the adjacent cross-gap recombination zone. The innovation also provides for novel circuits and unique applications, particularly for water sterilization.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/398,950 entitled “Gallium Nitride Cross-GapLight Emitters Based on Unipolar-Doped Tunneling Structures” filed onSep. 23, 2016, the entirety of which is incorporated by referenceherein.

ORIGIN OF THE INVENTION

The present application has Government rights assigned to the Office ofNaval Research under the “III-N Devices and Architectures for TerahertzElectronics” MURI program (N00014-11-1-0721) and its extension(N00014-16-1-2686).

TECHNICAL FIELD

The present innovation relates generally to gallium nitride baseddevices and, more particularly to the generation of holes in galliumnitride based devices lacking p-type doping, and their use in lightemitting diodes and lasers, both edge emitting and vertical emitting. Bytailoring the intrinsic design, a wide range of wavelengths can beemitted from near-infrared to mid ultraviolet, depending upon the designof the adjacent cross-gap recombination zone. The innovation alsoprovides for novel circuits and unique applications, particularly forwater sterilization.

BACKGROUND

Since the announcement of the first strong GaN blue-color,light-emitting diodes (LEDs), the interest in GaN photonics andelectronics has grown steadily, and the commercial applications haveexpanded to the extent that GaN devices now comprise a viable industry.The key step forward was the development of a high-quality p-type GaNepitaxial layer using Mg as a dopant and an AlN buffer layer on asapphire substrate. While the n-type dopant Si in GaN manifests as ashallow donor (˜15 meV), p-type dopants, such as Mg in GaN, manifest asmuch deeper acceptors (˜160 meV). The sapphire substrate was usedinstead of a GaN substrate because high-quality GaN substrates were notavailable at that time. Nevertheless, it allowed the growth of atraditional p-n (homo) junction LED having qualities similar to thosedemonstrated in GaAs since the 1960s. An exemplary conduction-valenceband-bending plot is shown in FIG. 1(a). The LED emission wavelength andexternal quantum efficiency (QE) were around 430 nm (violet: 380-450 nm)and 0.2%, respectively, the latter being much higher than the QEsreported for previous blue-emitting LEDs based on SiC and ZnSe LEDs.However, the peak wavelength was considerably longer than the expectedcross bandgap (U_(G)=3.4 eV) wavelength of 360 nm. This indicated thatthe recombination was via impurity states, likely associated with thep-type Mg dopant.

Soon thereafter GaN-based quantum-well LEDs were demonstrated usingInGaN for the quantum well. Although the external QE was only 0.15% andthe emission wavelength was still in the violet at 415 nm, this wasconsidered a major step forward since the use of a quantum well allowsfor tuning of the emission wavelength through control of the In fractionand the well width. GaN/InGaN quantum well LEDs were demonstratedranging in peak emission wavelength from blue around 450 nm(range=450-495 nm) to red at 675 nm (range=620-740 nm). In addition, forthe blue emitters an external QE of 20% was achieved.

The GaN/InGaN LED development segued quickly into the GaN laser diode(LD), demonstrated first in 1996. The gain medium consisted of multipleInGaN quantum wells and the lasing wavelength was near 404 nm (violet).However, the laser cavity consisted of the traditional in-planedouble-cleaved-facet structure, so the emission occurred in the sameplane as the quantum wells, not the more desirable vertical direction.So researchers pursued the vertical cavity, surface emitting laser(VCSEL) diode. However, the same issue that plagued GaN LEDs from thebeginning—the high resistivity of the p-doped GaN (e.g., Mgdopants)—again became a problem. This is because LDs of all typesgenerally run at higher electrical current levels than LEDs since morecurrent is required across the p-n junction to generate theelectron-hole population inversion necessary for lasing action. Theresistive p-doped layer not only causes a significant voltage drop andJoule heating, but it also creates a non-uniformity in the electricpotential which is deleterious to the laser efficiency. This is inaddition to the fact that the p-type GaN is generally difficult to grow,requiring extra materials (e.g., magnesium dopant) and processing (e.g.,high-temperature rapid thermal annealing to activate), which also addscost and reduces the yield in fabricating both LEDs and LDs alike.

The p-doping challenge has led researchers to unusual methods tomitigate the p-doping issues. To achieve population inversion anduniform light output across a LED to mitigate current crowding, theelectrical pump current needs to be uniformly spread over the p-GaNcontact area, starting from ohmic contacts located outside the opticalcavity defined by the DBR mirrors. The high resistivity of the p-GaNregion becomes a bottleneck for uniform carrier spreading. To overcomethis problem, a thin highly conductive ITO layer was introduced toreduce the resistance. However, the ITO layer adds additionaldifficulties in deposition and fabrication, and can contributenon-negligible loss to the optical cavity, which leads to higherthreshold current.

Independent of the detail design, all conventional p-n-junctionGaN-based light emitters suffer from a phenomenon called current“droop”. This occurs in devices designed for intense light emission,whereby the emission strength and internal quantum efficiency fall withincreasing current density above a certain level. The physical reason isthe poor mobility and high resistivity of the holes in p-doped GaN. Thiscauses the p-doped regions to heat up with increasing current density,which in turn increases the resistivity further and causes a significantfraction of the bias voltage to drop across the p-type region.

SUMMARY OF THE INVENTION

The following presents a simplified summary of the innovation in orderto provide a basic understanding of some aspects of the innovation. Thissummary is not an extensive overview of the innovation. It is notintended to identify key/critical elements of the innovation or todelineate the scope of the innovation. Its sole purpose is to presentsome concepts of the innovation in a simplified form as a prelude to themore detailed description that is presented later.

According to an aspect, the innovation provides viable hole generationfor radiative cross-gap recombination with electrically injectedelectrons. This eliminates the need for a p-n junction altogether andprovides for efficient hole generation by interband (Zener) tunneling ofelectrons. The radiative recombination with electrons can take placeeither in a quantum well if there are two or more barriers, or in theaccumulation regions for electrons and holes on the emitter side of thestructure (recombination zone).

According to an aspect, the innovation comprises a valence-to-conductioninterband electron tunneling diode, comprising: a substrate; an n-typebottom contact; a radiative recombination zone that could be a singlelayer, a single quantum well or a series of multi-quantum wells; abottom spacer; an electron barrier, either single or multiple barrier;an interband-tunneling hole generator which creates a largeconcentration of holes on the emitter side; a top spacer separating thetunneling region from the ohmic contact; and an n-doped top contactlayer.

In one embodiment, the structures according to the innovation cangenerate a high conduction-band electron current density through designof the heterobarriers and doping profiles. They can also generate a highdensity of holes. Without being bound by theory, the generation of highdensity holes may be principally by Zener tunneling of electrons, butpossibly also by impact ionization of valence-band states in thepresence of energetic conduction-band electrons. Because the electronand hole currents and densities are created by fundamentally differentphysical mechanisms, they can in principle be balanced. This is animportant consideration for efficient operation of any light emitter, beit an LED or LD. The balance between the two mechanisms depends on thedetailed GaN/InGaN/AlGaN/AlN heterostructure and doping profile. This isa first in GaN device technology.

In one example embodiment of the innovation, resonant-tunnelingconduction-band electron current densities of order 1×10⁴ A/cm², andZener tunneling densities of order 10² A/cm² have already been achievedin the baseline device (See FIGS. 2, 9 and 10). In one embodiment, theelectron current density could be decreased to match the Zener tunneling(or impact ionization) density, thereby enabling a much more efficientLED or LD. In another embodiment, a device according to the innovationcould increase the Zener tunneling current to match the electronconduction-band current, but this is more challenging. If the Zenertunneling were to increase significantly, say by one order-of-magnitude,the structure according to the innovation could be used as a holegenerator in conjunction with other device structures requiring holes incertain regions but electrons in others. This would allow for theelimination of a p-type contact, which is a difficult task inIII-nitrides.

According to an aspect, the innovation provides a solid-state devicecomprising a bottom n-type layer; a top n-type layer; a middle layerinserted between the top layer and bottom layer. The middle layer mayinclude at least two materials provided between the top and bottomlayers which serve as heterojunction tunnel barriers. The top layer andthe middle layer form an interband tunnel barrier to generate holes byZener tunneling across the potential barrier of the forbidden energygap, and where the middle layer forms at least one intraband tunnelbarrier to control electron flow.

In one embodiment, the innovation includes a device wherein the top,middle and bottom layers are comprised of gallium nitride, aluminumnitride, indium nitride or alloys and combinations of III-nitridesemiconductors or III-nitride compatible semiconductors. In oneembodiment, the heterojunction interband tunnel barrier is formed by thepolarization effects at III-nitride heterojunctions.

According to an aspect, the innovation provides a light emitting diodecomprising a bottom n-type layer; a top n-type layer; a middle layerinserted between the top layer and the bottom layer. The middle layermay comprise at least two materials provided between the top and bottomlayers which serve as heterojunction tunnel barriers. In one embodiment,the top, middle and bottom layers are independently selected fromgallium nitride, aluminum nitride, indium nitride or alloys andcombinations of III-nitride semiconductors or III-nitride compatiblesemiconductors.

The middle layer form an interband tunnel barrier to generate holes byZener tunneling across the potential barrier of the forbidden energygap, and where the middle layer forms a least one intraband tunnelbarrier to control electron flow. The radiative recombination of Zenerinjected holes from the top layer occurs directly with electronselectrically injected from the bottom layer.

In one embodiment, p-type doping is not part of the active device.

According to an aspect, the innovation provides a laser diode comprisinga bottom n-type layer; a top n-type layer; a middle layer insertedbetween the top layer and bottom layer, where the middle layer comprisesat least two materials provided between the top and bottom layers whichserve as heterojunction tunnel barriers. The top layer and the middlelayer form an interband tunnel barrier to generate holes by Zenertunneling across the potential barrier of the forbidden energy gap. Inaddition, the middle layers form at least one intraband tunnel barrierto control electron flow and wherein the radiative recombination ofZener injected holes from the top layer occurs directly with electronselectrically injected from the bottom layer. A Fabry-Perot etalon isadded external to the radiative recombination zone to form a laser diode

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A)-1(C) depict various stages of development of GaN-basedphotonics and electronics. FIG. 1. (a) Conduction- andvalence-band-bending diagram for homojunction GaN-based p-n+ LED showingphoton emission by GaN-cross-gap recombination on the p-side of thejunction. (b) Double-heterojunction, quantum-well laser showing photonemission between 2D quantized states in the InGaN quantum well, andtherefore at photon energy below the GaN bandgap. (c) Prior art MQW LEDwith a p-n junction clearly shown under bias.

FIGS. 2A and 2B depict design features and analysis of band bending.FIG. 2. (a) Epitaxial-layer “stack” for GaN-based, unipolar-doped RTDand LED structure. (b) Conduction-band bending diagram with zero biasapplied between emitter and collector. Note: the bottom epitaxial layerof FIG. 3(a) is the “emitter” of FIG. 3(b).

FIGS. 3A and 3B are graphs depicting the results of interband-tunnelinganalyses. FIG. 3. (a) Conduction- and valence-band bending in GaN/AlNRTD structure computed numerically with Silvaco Atlas at a bias voltageof 5.0 V. (b) Hole generation rate (per unit volume) for bulk GaN as afunction of electric field, assuming a bandgap energy of 3.4 eV and areduced effective mass of m_(r)=0.20m₀.

FIGS. 4A and 4B depicts design features and analysis of aresonant-tunneling diode according to the innovation. (a) Crosssectional view of GaN RTD structure showing the same “stack” as 3(a) butwith the metallization, mesa isolation, and planarization aspects. (b)Current-density-vs-voltage curves for mesas in (a) ranging in areabetween 12 and 96 square microns.

FIGS. 5A and 5B are a photograph and analysis of an RTD structureaccording to the innovation. FIG. 5. (a) Photograph of GaN RTD structureshowing the three DC-coupled electrodes, and the RTD mesa, easilyidentified through its emission of violet light. (b) Light spectralintensity curves vs bias voltage showing a dominant peak centered at≈360 nm. The insert plot shows the emission spectrum under reverse bias.The measurements were made with a fiber grating spectrometer having aresolution of 2.0 nm.

FIGS. 6A and 6B are graphs depicting the light-vs-bias voltage (L-V) andlight-vs-bias current (L-I) curves for the device of FIGS. 5A and 5B.(a) the light-vs-bias voltage (L-V) and (b) light-vs-bias current (L-I)curves.

FIG. 7 is a graph depicting the current-voltage curves for the GaN/AlNRTD structure of FIGS. 5A and 5B. The experimental curve is fit using acombination of resonant-tunneling and leakage-current models for theelectrons, and a Zener tunneling model for the holes. The sum of themodel currents is a good fit to experimental curve.

FIG. 8 is a graph depicting the same curves as FIG. 7 but plottedlog-linear to see the relative magnitude of the electron and holecurrents more clearly. The model electron current is 250× greater thanthe hole current at a bias of 7.0 V.

FIG. 9 is a graph depicting simulated current-voltage curves for amodified structure having lower n-type doping on the emitter side toreduce the electron current density to a level closer (within 12×) ofthe hole current density.

FIGS. 10A and 10B depict example embodiments of unipolar doped resonanttunneling light emitting diodes according to the innovation.

FIGS. 11A and 11B depict example embodiments of unipolar doped resonanttunneling light emitting diodes according to the innovation.

FIG. 12 depicts an example embodiment of unipolar doped resonanttunneling light emitting diodes according to the innovation.

FIG. 13 depicts an example embodiment of unipolar doped resonanttunneling light emitting diodes according to the innovation.

FIGS. 14A-14C depict example embodiments of unipolar doped resonanttunneling light emitting diodes according to the innovation.

FIGS. 15A-15B depict example embodiments of unipolar doped resonanttunneling light emitting diodes according to the innovation.

FIG. 16 depicts an example embodiment of unipolar doped resonanttunneling light emitting diodes according to the innovation.

FIG. 17 depicts an example embodiment of unipolar doped resonanttunneling light emitting diodes according to the innovation.

FIG. 18 is an illustration depicting a bipolar GaN-based VCSEL incross-section. A p-doped GaN region is required for hole carrierinjection, while a n+ region is for electron injection. A thin ITO layeris located next to the proximity of the p+ GaN region to facilitate holecarrier spreading.

FIG. 19 is an illustration depicting a unipolar-doped VCSEL structure incross-section. Both contact regions are n+ type, which has much higherconductivity than any p-type GaN currently known. Thus no ITO layer isnecessary as in the p-n VCSEL of FIG. 2.

FIGS. 20A-20C depicts a diagram and analyses of an RTD structureaccording to the innovation. (a) Top view of RTD coupled to a coplanartransmission line to make a relaxation oscillator. (b) I-V curve of RTDundergoing relaxation oscillations with four characteristic pointsillustrated. (c) Voltage waveform across RTD showing the same fourpoints as in (b), and the impulsive nature that results from the fastRTD switching.

FIGS. 21A and 21B depict analyses of the same RTD structure as in FIG.20A. (a) The same RTD I-V curve as in FIG. 20B superimposed with the L-Vcurve assuming that the threshold voltage of the light emission is at ornear the peak voltage of the RTD NDR region. (b) Same RTD relaxationoscillator waveform as in FIG. 20C superimposed with the correspondinglight emission waveform, resembling an ideal optical clock.

FIGS. 22A and 22B are graphs representing sterilization effectiveness ofan embodiment according to the innovation. a) E-coli germicidal spectraleffectiveness curve compared to typical UV-C LED emission spectrum UV-CLED emitting 265 nm compared to E. coli germicidal effectiveness curve.(b) Absorption coefficient of clear water with the UV-C sterilizationband superimposed.

FIGS. 23A and 23B are cross-sectional views of unipolar-doped bipolartunneling LED. (a) Cross-sectional view of unipolar-dopedbipolar-tunneling LED showing a transparent optical contact on top forincreased EQE. (b). Same as (a) except for the addition of a reflectinglayer near the bottom of the Al_(x)Ga_(1-x)N active layer and consistingof GaN quantum dots.

FIG. 24 is a perspective view of a 2D array of LEDs such as those ofFIG. 19 in the form of parallel stripes each emitting UV-C photonsprimarily in the vertical direction.

FIGS. 25A and 25B depicts a diagram showing a) A method for configuringthe UV-C LED array of FIG. 24 to a tube for the purpose of watersterilization. (b). A method for coupling the LED array to beamcollimating optics for the purpose of hygiene surface and objectsterilization.

DETAILED DESCRIPTION

The innovation is now described with reference to the drawings, whereinlike reference numerals are used to refer to like elements throughout.In the following description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the subject innovation. It may be evident, however,that the innovation can be practiced without these specific details. Inother instances, well-known structures and devices are shown in blockdiagram form in order to facilitate describing the innovation.

According to an aspect, the innovation provides an alternative approachto GaN-based light emitters that does not require p-type GaN or a p-njunction at all. In one embodiment, the innovation provides a devicethat is the unipolar-doped (n-type only) GaN resonant tunnelingstructure shown in FIG. 2(a). All the epitaxial layers are grown onsemi-insulating or n-doped Ga-polar GaN, N-polar GaN, sapphire, SiC,AlN, or Si substrates by either radio frequency (RF) plasma assistedmolecular beam epitaxy (MBE) or by metalorganic chemical vapordeposition (MOCVD). The polarization changes direction between Ga- andN-polar GaN, which allows for the entire device structure to be flipped.This allows for greater control over where the recombination zone willbe located, and therefore where the light will be emitted. In oneembodiment, the GaN substrate has a smooth surface morphology and lowdefect concentration. It is understood that the epitaxial layers of the“active region”, numbered 10-50 in FIG. 2(a) could be reversed in orderwith little or no effect on the device performance.

According to the innovation, the first epitaxial layer is doped heavilyn⁺⁺-type (>1×10¹⁹ cm⁻³) with silicon to serve as the bottom ohmiccontact layer after the isolation of individual devices. The next fiveepitaxial layers above the n⁺ are unintentionally doped (UID), but havea finite critical thickness larger than the n⁺⁺ region for which itbecomes energetically favorable to produce dislocations to reduce localstrain caused by lattice mismatch of the epilayers as referenced to thesubstrate. The mismatched layers used here are well below that criticalthickness, which is the thickness at which defects spontaneously occur.The thicknesses shown in FIG. 2(a) are specific to the design that hasalready been demonstrated as a resonant-tunneling diode (RTD) havingstable negative differential resistance (NDR), and as a unipolar-dopedLED (see below). The NDR is made possible by electron tunneling throughthe double-barrier “heterostructure”: two AlN barriers, each nominally2.0 nm thick, separated by a GaN quantum well, nominally 3.0 nm thick.Other thicknesses may work for LED or LD emission as well.

Epitaxial Growth

In one embodiment of the innovation, the growth technique of the GaNunipolar-doped LEDs is RF-plasma assisted molecular beam epitaxy (MBE)on freestanding, Ga-polar (0001) GaN substrates. Suitable GaN substratesmay be grown by hydride vapor phase epitaxy (HVPE). A high-quality HVPEGaN substrate wafer is estimated to have a density of threadingdislocations between 10⁶ and 10⁷ cm⁻². The Ga-polar (0001) surface ofthe wafer is finished with a chemical-mechanical polish (CMP). Prior toepitaxial growth, the HVPE GaN wafer should be prepared in theultra-high vacuum (UHV) MBE system using a wet chemical clean. In oneembodiment, after loading into the MBE system the substrate may beout-gassed under UHV conditions for 30 minutes at 600° C. After coolingto room temperature, the substrate may be transferred to the depositionchamber for growth.

In one embodiment, the deposition chamber may be equipped withdual-filament effusion cells for evaporation of elemental aluminum andgallium; a single-filament effusion cell for evaporation of elementalsilicon, used as an n-type dopant; and an RF plasma source for deliveryof nitrogen. The substrate temperature may be measured by a thermocouplemounted behind the substrate and is maintained at a constant temperaturein the range of about 700° C. to about 900° C. throughout the growth. Inone embodiment, the temperature is maintained at approximately 860° C.

In one embodiment, the plasma may be operated at a constant power ofroughly 300 W with N₂ gas flow of roughly 0.9 standard cubic centimetersper minute (sccm). The gallium and aluminum fluxes can be chosen tocreate nitrogen-limited growth conditions with no accumulation of excessmetal and a growth rate of approximately 3 nm/min. The silicon effusioncell conditions can be adjusted to result in a silicon concentration ofapproximately 5×10¹⁹ cm⁻³ in the GaN buffer (lower contact) and 8×10¹⁹cm⁻³ in the upper GaN contact layer.

In one embodiment, growth may be initiated with a 2-min exposure of thesubstrate surface to the N plasma, followed by simultaneous opening ofthe Ga and Si shutters. All layers should be grown continuously andwithout interrupts. Growth should be terminated by the closing of allshutters, extinguishing the N plasma, and cooling the sample to roomtemperature while in the MBE system

Device Physics

The doping concentration is an important design feature for the devicephysics. In one embodiment, the plasma-assisted MBE growth allows forvery high concentrations of n-type doping (e.g., up to ≈10²⁰ cm⁻³). Thisallows for a heavy accumulation of free electrons on the emitter side ofthe barriers (see bottom side in FIG. 2(a)) when the device is underforward bias (positive voltage on the top contact). The high electronconcentration promotes high electron tunneling current density. The highn-type doping also enables ohmic contacts on the top and bottom of thestructure having very low contact resistance. This makes the device moreefficient as a light emitter and a negative-resistance oscillator, andreduces contact-induced heating too.

According to another aspect, the innovation provides band bending, i.e.,the variation of the conduction- and valence-band electron potentialenergies with location in space, and under bias voltage. The zero-biasband bending diagram is shown in FIG. 2(b). This is non-trivial in GaNbecause of two important effects: (1) electric polarization, and (2)piezoelectricity. These effects normally do not significantly affectmore common semiconductor-heterostructure materials such as GaAs/AlAs orInGaAs/AlGaAs. The difference is that GaN, AlN, and Al_(X)Ga_(1-X)Nalloys thereof, are all highly non-centrosymmetric (hexagonal Wurtzitecrystal structure) with a strong built-in dipole moment per unit cell.Because the dipole moment is different in GaN and AlN, there is adiscontinuity in the electric polarization P at the interfaces betweenthe two materials, which from classical electrostatics creates a boundsheet-charge.

A piezoelectric effect occurs because of the strain induced in the thinAlN barriers by the tendency for thin crystalline layers to match theirlateral lattice constants to that of the crystalline material in whichthey are embedded (commonly called “pseudomorphic” matching in theliterature). For thin AlN barriers embedded in GaN, this means that thebasal lattice constant of AlN, a=0.311 nm at 300 K, matches to the basallattice constant of GaN, a=0.319 nm at 300 K. In other words, the AlNmust expand laterally by ≈2.5% which creates elastic tension and aresultant perpendicular electric field because of its largecross-coupled piezoelectric coefficient (piezoelectric stresscoefficient e₃₁=−0.60 Cb/m²). The result of both the polarization andpiezoelectric effects is a huge discontinuity in the electric field(i.e., −dϕ/dx where ϕ is the electron potential energy plotted in FIG.2(b)) at the interfaces. Such a strong discontinuity that the electricfield changes sign from positive to negative, or vice versa, at all fourof the GaN/AlN interfaces is remarkable.

Given the abrupt discontinuities shown in FIG. 2(b), somethinginteresting happens in the band bending: the electric field at the lastGaN/AlN interface relative to the emitter side [left-most interface ofFIG. 2(b)] has approximately the same magnitude and sign as at theright-most GaN/AlN interface. In other words, the strong polarizationand piezoelectric effects cancel out because of the even number (four)of heterointerfaces. Therefore, in an embodiment in which the field onthe emitter side is designed to be very large, then the field on thecollector side of the barriers will be equally large. As shown in FIG.2(b), the emitter field is very large because of the UID layerseparating the barriers from the Si-doped n⁺⁺ doped region. Electronsfrom the n⁺⁺ region diffuse into the UID region, creating an“accumulation” region with rapidly varying slope. This occurs even withzero bias applied to the device. The corresponding large field on thecollector side is then maintained by the UID layer on that side (a“depletion” region), creating a very large drop in electron potentialenergy (˜2.0 eV), even at zero bias.

This unusual band bending in the GaN/AlN RTD structure creates thecapability for cross-gap light emission from electron-hole pairs withthe holes created through valence-to-conduction interband (Zener)tunneling. The hole generation occurs at the point in the structurewhere the valence band electrons starts the interband tunneling process.To understand how interband tunneling can occur in such a wide-bandgapmaterial, plotting of the bivalent band bending, conduction and valencebands together, as shown in FIG. 3(a), at a bias voltage [applied to thetop contact of FIG. 2(a)] of +5.0 V. This band-bending diagram wascomputed using a numerical transport tool, Silvaco-Atlas, which accountsfor all electrostatic, polarization, and piezoelectric effects, as wellas drift-diffusion, semiclassical, and quantum-transport effects. Thevertical axis is still electron potential energy as in FIG. 2(b), andthe horizontal axis is the distance from the quasi-neutral point of thetop n⁺⁺ region of the device structure of FIG. 2(a). The valenceband-edge in the GaN at the collector edge of the double-barrierstructure is only about 10 nm separated from the conduction band edge inthe GaN at the left end of the UID region on the collector side. This isbecause of the very large electric field across this region. And itmeans that there may be a significant probability of interband tunnelingof electrons from the valence band to the conduction band, as depictedsymbolically with the red horizontal arrow in FIG. 3(a). This is knownhistorically as Zener tunneling and has been observed in many differentsemiconductors including Si. Thus, holes are generated inside theLED/laser, instead of injected from the outside resistive contacts andbulk layers.

The interband-tunneling hole generation rate using the venerable modelof E.O. Kane was also computed. The result, plotted in FIG. 3(b), showsthe very rapid rise of hole-generation rate with field. From the slopein the depletion layer of FIG. 3(a), the electric field was estimated tobe E≈U_(G)/8 nm=(3.4 eV)/(8 nm)→4.2×10⁶ V/cm—a huge field bysemiconductor standards but still less than the accepted breakdown fieldin GaN of 5×10⁶ V/cm. This value of E field is superimposed on the Kanecurve in FIG. 3(b), showing that the hole generation rate is≈10¹³/cm³-s. FIG. 3(a) shows where these holes are created, in the GaNadjacent to the AlN barrier on the collector side. Without being boundby theory, it is hypothesized that these holes then tunnel through tothe GaN electron-accumulation layer on the emitter side, as showngraphically in FIG. 3(a). Once on the emitter side, they can recombineradiatively with electrons because of the direct bandgap of GaN. Thistwo-step tunneling process is aided by the fact that the holes createdon the collector side by the first (Zener tunneling) step have virtuallyno electrons to recombine with in this same region. They primary meansof relaxing in (kinetic) energy is to tunnel through the two AlNvalence-band barriers to the emitter side, where there is a high densityof electrons for joint radiative recombination.

This hole generation and tunneling enables another, more specificradiative recombination depicted graphically in FIG. 3(a) inside thequantum well. In the process of holes tunneling through the barriersfrom the collector side toward the emitter side, they could possiblydwell at one of the quantum-confined hole levels in the quantum well,such as the third hole level in FIG. 3(a). It is well known thatresonant tunneling through upper levels in a double-barrier quantum wellcan populate the lower levels by energy relaxation via phonon emission.This includes the ground level, E_(1h), shown in FIG. 3(a). The same istrue of the majority carrier (electron) resonant tunneling through thequantum well of FIG. 3(a): electrons tunnel through an upper level(2^(nd) level) but by so doing, can populate the ground state E_(1e)too. Given simultaneous populations of electrons in E_(1e) and holes inE_(1h), cross-gap radiative recombination is possible between thequantum-confined states.

Quantum-well cross-gap recombination is well known in GaAs, InGaAs, andGaN materials too. Significantly, according to the present innovation,the electric field across the GaN quantum well is enormous compared tothese other materials because of the very high doping, and thepolarization and piezoelectric effects. This introduces a very largeStark (red) shift in the emitted photon hν_(p) compared to what it wouldbe at zero bias. This is called the quantum-confined Stark effect(QCSE). The electron wavefunction becomes concentrated in the collectorside of the quantum well, and the hole wavefunction in the emitter side,and the Stark-shifted photon energy is roughly.

hν _(p) =U _(G) +U _(1e) +U _(1h) −E·w

Here, E is the electric field, assumed uniform across the quantum well,and w is the width of the well. Given E_(W)˜4.2×10⁶ V/cm, and w=2 nm(2×10⁻⁷ cm), ΔU=0.84 eV. This is greater than U_(1e)+U_(1h) in FIG.3(a), so that under these conditions hν_(p) is less than U_(G) (i.e.,λp>370 nm). Based on the ground levels shown in FIG. 3(a), hν_(p)≈2.75eV (λ_(p)≈450 nm), which is in the violet region of the visiblespectrum.

Experimental Demonstration

The first demonstration of the unipolar-doped, light emitting structureof FIG. 2(a) was purely electrical, testing for the tell-tale metric forRTDs—the negative differential resistance (NDR). First, RTDs werefabricated as shown in the cross section of FIG. 4(a). The metalcontacts were fabricated directly on the top Si-doped n⁺⁺ layer of FIG.2(a), and mesas were defined by dry etching using the top ohmic contactas an etch mask. Then the structures were isolated with SiO₂, and viaholes were fabricated to allow ohmic contacts to the bottom n⁺⁺ layer.Shown in FIG. 4(b) are the resulting room-temperature current-density vsvoltage (J-V) curves of RTD structures having different mesa area. Theresulting J-V curves display an unmistakable NDR region with a peakvoltage close to 4.0 V and a valley voltage around 4.5 V. Each of theJ-V curves was highly repeatable, and although the peak-to-valleycurrent ratio was not outstanding by common RTD standards, these resultsare the best that have ever been obtained in GaN-based RTDs in over adecade of investigation by researchers worldwide.

When the bias voltage was increased above ˜4.5 V, a very interestingphenomenon occurred. The RTD mesa began emitting bright violet-coloredlight around its periphery as seen in the photograph of FIG. 5(a) [takenwith a cellphone camera], which also shows the three electrodes(ground-signal-ground configuration) used to apply the electrical bias.The light emission was bright enough to be easily detected by afiber-coupled grating spectrometer. The emission spectrum is shown inFIG. 5(b) with a dominant peak centered around 360 nm that increases inintensity with increasing bias current, and has a full-width at halfmaximum (FWHM) of 16 nm. 360 nm is, in fact, the wavelengthcorresponding to the 3.4 eV band-gap of GaN at room temperature. Thissupports the hypothesis that light emission is created by therecombination of electron-hole pairs in the electron accumulation regionon the emitter side as shown schematically in FIG. 4(a) and explainedabove in the text. The insert in FIG. 5(b) shows a similar emissionspectrum under reverse bias although peaked at 365 instead of 360 nm.This further supports the hypothesis that the light emission is createdby the recombination of electron-hole pairs across the GaN band-gap, butthe emission spectrum does depend on asymmetry in the doping profile.

By measuring the peak intensity and normalizing to the backgroundlaboratory light, the light-vs-bias voltage (L-V) and light-vs-biascurrent (L-I) curves shown in FIGS. 6(a) and 6(b) were constructed,respectively. Both exhibit a threshold effect, the L-V curve at ≈4.5 V,and the L-I curve at ≈5.0 mA. Well above threshold, the L-I curveapproaches the linear behavior expected for LEDs and LDs both. But asseen in the spectral emission curves of FIG. 5(b), the FWHM remainsbetween 21 and 22 nm, even at the highest bias levels applied. This isconsidered too broad for a laser emission, so it was concluded that thedevice was operating as an LED, not an LD. Further evidence to supportthis is that there is no intentional optical cavity for this fabricateddevice structure of FIG. 4(a) necessary to support laser action.Although the metal contact to the top n++ layer could be acting as amirror, there is no parallel mirror on the bottom of the structuredirectly below the top metal contact. Instead the metal contacts to thebottom layer are outside the area of the mesa and accessed through thevia holes shown in FIGS. 4(a). Clearly, a re-design of the fabricationdesign and process flow, leveraging the intrinsic novel epilayer design,could be performed to add an appropriate external cavity.

Designing for Higher Quantum Efficiency in GaN LEDs

The present RTD/LED structure was designed for good RTD behavior,especially stable NDR at room temperature, not for efficient near-UVemission. Investigations were also conducted for simple ways ofincreasing the near-UV external quantum efficiency, even if deleteriousto the RTD performance. The simplest so far is to bring the electron andhole current densities closer to equality by changing the devicematerial parameters. To do this with confidence, analytic models of theelectron and hole currents using standard formulations were developed.For the electrons, the inelastic form of the Breit-Wigner transmissionprobability was used through a single quasibound level in the presenceof scattering, and integrate it over the Fermi-sea on the emitter sideusing the standard Tsu-Esaki integral of quantum transport theory. Anelectron “leakage” current term to represent a combination of: (1)inelastic tunneling at longitudinal energies well away from thequasibound level, and (2) thermionic emission over the top of thebarriers was added. The leakage term has the form of the Shockleyequation, I_(L)=I₀[exp(αV/k_(B)T)−1], where I₀ and α are constantsdetermined by curve-fitting to the experimental data.

The hole current is associated with Zener (i.e., cross-gap) tunnelingacross the GaN bandgap in the depletion layer on the collector side ofthe AlN double-barriers. This mechanism is enabled by the huge electricfield in this region created by the bias voltage along with thepolarization- and piezoelectric-induced surface charge densities on allfour GaN/AlN interfaces. A k-dot-p approach is used to evaluate thecurrent density using a WKB approximation for the tunneling integral.

FIG. 7 compares the experimental J-V curve against the electron and holecurrent models for a bias voltage of 2.0-6.0 V, all plottedlinear-linear. The combination of resonant-tunneling and leakage currentof the electrons provides a good fit to the experimental J-V, the holecurrent being so much smaller in comparison that it essentiallycoincides with the abscissa in this plot. To ascertain the relative sizeof the hole current, FIG. 8 shows the experimental J on a log_(e)-linearplot, an up to 8.0 V bias. Above ˜5.0 V near where the device displays athreshold in near-UV light emission, the hole current density is belowthe electron current density (which is mostly leakage current) by ≈250times, meaning at best the holes combine with 1/250^(th) of theavailable electrons, thus effectively capping the internal efficiency inthis early prototype. Clearly, this poses an efficiency problem since itmeans that even if all holes recombined with electrons radiatively andall the emitted photons were collected without loss, the highesttheoretical internal QE would be 0.004%.

Consequently, Applicant's approach to increasing the internal QE is toshrink this difference. Further investigation of the models in describedabove (see paragraph [0032] revealed that the simplest way to do this isto reduce the electron current density while holding the hole densitynearly constant. A reduction in the Fermi energy (E_(F)) on the emitterside does exactly this, the electron resonant-tunneling and leakagemechanisms both falling monotonically, while the Zener-tunneling ofholes having practically no dependence on E_(F) on the emitter side atall. The n-type doping concentration outside the spacer layer on theemitter side determines E_(F), and for the existing structure withN_(D)=5×10¹⁹ cm⁻³, E_(F)=0.25 eV using the conduction-band parameter ofGaN, m*=0.20 m_(e). A reduction of N_(D) to 5×10¹⁸ would drop E_(F) to0.05 eV, and the resulting model J-V curves are plotted in FIG. 9. Theelectron current drops dramatically, but not the hole current, so thatthe difference between the two is just 12.0× at 7.0 V bias and 8.9× at8.0 V bias. In other words, this relatively simple change in materialgrowth parameter should improve the internal QE by at least 25×, allother material parameters assumed unchanged.

Device Designs

Examples of embodiments according to the innovation are provided in thedrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts. Allembodiments are incumbent on a GaN/Al(Ga)N hole generator thateliminates the need for any p-type doping or p-type contacts.

Double Barrier RTD-LED Baseline Design (Platform)

According to an aspect, the innovation includes a new platform for solidstate light emission. This new platform for solid state light emissionthat combines unipolar doping with bipolar tunneling for creating holesat, or immediately adjacent to, the zone of recombination with a genericnitride LED or laser has been designed, built and tested. Overcoming theobstacle of hole injection completely is a significant advancementproposed herein. And this embodiment can take on a myriad ofpermutations of design architectures.

For the unipolar doping with hole injection to occur, there exists arange of layer thicknesses, layer doping levels, etc. that provides forenough band bending. For instance, FIG. 10(a) shows the proposedunipolar doped resonant tunneling light emitting diode that injectsholes across layer 80 into a single emission zone, in or near layer 100.This design uses a double-barrier RTD injector. It will be appreciatedthat this could be replaced with a single barrier design. In thisembodiment, there are acceptable ranges for multiple parameters acrossthe structure—these ranges and parameters are listed in the Table I.Layer 90 in FIG. 10(a) is composed of several additional layers shown inthe exploded view of FIG. 10(b). From FIG. 10(b), layers 91-93 haveacceptable ranges for multiple parameters across the structure—theseranges and parameters are listed in Table I.

TABLE I Material Characteristics FIG. 10(a) - UDRT-LED top contactregion n-type doping concentration: 5E16-2E20 cm⁻³ top contact regionthickness: 10-500 nm top contact region composition: GaN, InGaN (1%-30%In), AlGaN (1%-90% Al) UID top spacer thickness: 2-15 nm UID top spacercomposition: GaN, InN, InGaN (1%-99% In) UID bottom spacer thickness:1-50 nm UID bottom spacer composition: GaN, InGaN (1%-100% In), AlGaN(1%-90% Al) bottom contact region n-type doping concentration: 5E16-2E20cm⁻³ bottom contact region thickness: 10-1000 nm bottom contact regioncomposition: GaN, InGaN (1%-30% In), AlGaN (1%-90% Al) substrate: GaN,Sapphire, AlN, SiC, Si FIG. 10(b) - Double barrier for 10(a) UID topbarrier composition: AlN, AlGaN (40%-99% Al) UID top barrier thickness:0.5-5 nm UID quantum well composition: GaN, InN, InGaN (1%-99% In),AlGaN (1%-70% Al) UID quantum well thickness: 1-8 nm UID bottom barriercomposition: AlN, AlGaN (40%-99% Al) UID bottom barrier thickness: 1-8nm

FIG. 11(a) shows another embodiment according to the innovation which issimilar to FIG. 10(a) but has multiple quantum wells on the emitter sidelabelled as 170. This design is also scalable, meaning that modificationof the recombination zone by adding multiple quantum wells (MQW)generally increases the radiative cross section for electron-holerecombination, and narrows its spectral width. This is particularlyattractive for applications requiring a single narrow emissionwavelength. A challenge with the MQW approach is lattice matching. Thequantum wells will be mechanically strained so that only a limitednumber of them can be used before the strain exceeds the elastic limit,causing the material to “crack” at the microscopic level. This resultsin atomic dislocations and other defects in the material, which aregenerally deleterious to the operation of “bipolar” devices like theelectron-hole light emitters. The number of quantum wells alloweddepends inversely on the magnitude of strain, which depends in turn onthe In or Al fraction in the In_(x)Ga_(1-x)N or Al_(x)Ga_(1-x)N. Sincethe emission wavelength also increases with In fraction (throughreduction in the bandgap), the number of quantum wells allowed scalesinversely with the emission wavelength. So, more quantum wells will beallowed in a violet emitter than in a blue emitter, for example.

Additionally, as the number of QWs increases, so does the injectionefficiency. This is because the carriers have a greater chance ofrecombining radiatively with each additional QW rather than beingcollected by a contact. And each quantum well augments all measures ofthe QE by creating a strong overlap between the electron and holewavefunctions. So as the injection efficiency increases, the LEDexternal QE does as well. As mentioned above, the emission wavelengthcan be controlled by modifying the recombination zone of the devicedisplayed in FIG. 11(a) via In or Al fraction and quantum well width.This is similar to the design strategy in the prior art example of FIG.1(c), but with the novel replacement of the traditional p-type layerwith that of the unipolar doped bipolar tunneling design. The embodimentdisplayed in FIG. 11(a), has acceptable ranges for multiple parametersacross the structure—these ranges and parameters are listed in Table II.It will be appreciated that this proposed intrinsic design can betransformed from an LED topology, to a planar laser or vertical laserconfiguration too.

In the previous embodiment, layers 150 were described as a doublebarrier, shown here as FIG. 11(b). Here the double barrier is moreclearly identified through the ranges for layers 151, 152, and 153 aslisted in Table II.

TABLE II Material Characteristics FIG. 11(a) - MQW UDRT-LED top contactregion n-type doping concentration: 5E16-2E20 cm⁻³ top contact regionthickness: 10-500 nm top contact region composition: GaN, InGaN (1%-30%In), AlGaN (1%-90% Al) UID top spacer thickness: 2-15 nm UID top spacercomposition: GaN, InN, InGaN (1%-99% In) UID bottom spacer thickness:1-50 nm UID bottom spacer composition: GaN, InGaN (1%-100% In), AlGaN(1%-90% Al) UID MQW region Number of QWs: 1-10 QW composition: GaN, InN,InGaN (1%-99% In), AlGaN (1%-90% Al) QW thickness: 1-8 nm Number ofbarriers: 2-11 Barrier composition: GaN, AlN, AlGaN (5%-99% Al) Barrierthickness: 1-10 nm bottom contact region n-type doping concentration:5E16-2E20 cm⁻³ bottom contact region thickness: 10-1000 nm bottomcontact region composition: GaN, InGaN (1%-30% In), AlGaN (1%-90% Al)substrate: GaN, Sapphire, AlN, SiC, Si FIG. 11(b) - Double barrier for12(a) UID top barrier composition: AlN, AlGaN (40%-99% Al) UID topbarrier thickness: 0.5-5 nm UID quantum well composition: GaN, InN,InGaN (1%-99% In), AlGaN (1%-70% Al) UID quantum well thickness: 1-8 nmUID bottom barrier composition: AlN, AlGaN (40%-99% Al) UID bottombarrier thickness: 1-8 nm

Single Barrier RTD-LED Basic Platform

The GaN—AlN double-barrier light-emitter structure described in FIGS.2(a) and 3(a) and applied in FIGS. 10 and 11 is just one possible designusing the unipolar-doped, cross-gap concept according to the innovation.This is useful for some novel concepts, such as highly coherent opticalclocks described herein. But if the goal is just light emission, then asingle-barrier structure may suffice such as that shown in FIG. 12,which is generally easier to grow epitaxially and easier to design too.

One difference between a double-barrier and single-barrier structure isthe current density. Double barrier structures of barrier thickness,t_(double), generally provide much more current density than a singlebarrier structure of thickness, t_(single), at or near thedouble-barrier resonant bias voltage (i.e., where electrons injectedfrom the emitter side have kinetic energy equal to the binding energy ofone of the quasibound states in the quantum well between the twobarriers). However, well away from the resonant bias condition, thedouble-barrier structure will provide even less current density than thesingle-barrier structure. So, optimization of the single-barrierstructure for light emission may entail a different barrier thickness,probably thinner, than what has been used in a first demonstration withthe double-barrier light emitter, i.e., 2.0 nm as shown in FIGS. 2 and3.

An additional challenge is the charge balance between the forwardelectron current and the reverse (Zener tunneling generated) holecurrent. Mismatches will cause excessive carrier pooling, leading toshifts in the internal bias that would oppose carrier injection, similarto a bipolar junction transistor. A well-designed structure willincorporate a nearly 1:1 injection of electrons and holes at the optimalbias point. The Zener tunneling creates holes on the collector side thatmust then tunnel through the barriers to reach the recombination zone onthe emitter side. But in the second step the holes experience a muchsmaller barrier than the electrons, so this step will likely be muchmore probable than the Zener tunneling itself. Hence, the Zenertunneling on the collector side becomes the bottleneck for total holegeneration in the recombination zone. So, in addition tobarrier-thickness, spacer-thickness on the collector side and dopingprofile will likely be important design parameters.

In one embodiment, there are acceptable ranges for multiple parametersacross the structure—these ranges and parameters are listed in TableIII.

The recombination zone in and around layers 230 and 240 (FIG. 12) can betailored to specific emission wavelengths by the addition of quantumwells, like the design put forth in FIG. 11(a). FIG. 13 elucidates thisproposed design where layer 300 has now been inserted as part of therecombination zone to control wavelength emission. In this embodiment,there are acceptable ranges for multiple parameters across thestructure—these ranges and parameters are listed in Table III. Someoneskilled in the art will perceive that this proposed intrinsic design canbe transformed from an LED topology, to a planar laser or vertical laserconfiguration too.

TABLE III Material Characteristics FIG. 12 - Single barrier UDRT-LED topcontact region n-type doping concentration: 5E16-2E20 cm⁻³ top contactregion thickness: 10-500 nm top contact region composition: GaN, InGaN(1%-30% In), AlGaN (1%-90% Al) UID top spacer thickness: 2-15 nm UID topspacer composition: GaN, InN, InGaN (1%-99% In) UID single barriercomposition: AlN, AlGaN (40%-99% Al) UID single barrier thickness: 0.5-5nm UID bottom spacer thickness: 1-50 nm UID bottom spacer composition:GaN, InGaN (1%-100% In), AlGaN (1%-90% Al) bottom contact region n-typedoping concentration: 5E16-2E20 cm⁻³ bottom contact region thickness:10-1000 nm bottom contact region composition: GaN, InGaN (1%-30% In),AlGaN (1%-90% Al) substrate: GaN, Sapphire, AlN, SiC, Si FIG. 13 -Single barrier MQW UDRT-LED top contact region n-type dopingconcentration: 5E16-2E20 cm⁻³ top contact region thickness: 10-500 nmtop contact region composition: GaN, InGaN (1%-30% In), AlGaN (1%-90%Al) UID top spacer thickness: 2-15 nm UID top spacer composition: GaN,InN, InGaN (1%-99% In) UID single barrier composition: AlN, AlGaN(40%-99% Al) UID single barrier thickness: 0.5-5 nm UID bottom spacerthickness: 1-50 nm UID bottom spacer composition: GaN, InGaN (1%-100%In), AlGaN (1%-90% Al) UID MQW region Number of QWs: 1-10 QWcomposition: GaN, InN, InGaN (1%-99% In), AlGaN (1%-90% Al) QWthickness: 1-8 nm Number of barriers: 2-11 Barrier composition: GaN,AlN, AlGaN (5%-99% Al) Barrier thickness: 1-10 nm bottom contact regionn-type doping concentration: 5E16-2E20 cm⁻³ bottom contact regionthickness: 10-1000 nm bottom contact region composition: GaN, InGaN(1%-30% In), AlGaN (1%-90% Al) substrate: GaN, Sapphire, AlN, SiC, SiUnipolar-Doped with InGaN Pre-Well

The unipolar-doped interband-tunneling light emitter according to theinnovation, be it an LED or an LD, and whether having a single- ordouble-barrier structure, will display even stronger emission at violetor blue wavelengths through the use of the ternary In_(x)Ga_(1-x)Nalloys in the “pre-well” approach shown in FIG. 14(a). The addition ofIn with GaN generally reduces the direct bandgap significantly, which itwhy it has been used to make quantum wells in conventional p-n LEDs andLDs. And by adding an InGaN quantum well, the desired photon emissionenergy is not susceptible to self-absorption by the GaN cladding, thusincreasing external quantum efficiency. By adding it to the emitter sidein the structure, the electron-hole recombination will naturally occurat wavelengths longer than 360 nm, and more electrons will accumulatethere than without the In incorporation, so the electron current densityshould increase too.

Because In_(x)Ga_(1-x)N is also non-centrosymmetric and polar, it shouldsupport a strong polarization effect and piezoelectric response of thefirst AlN barrier. The primary challenge will be the lattice mismatchbetween the In_(x)Ga_(1-x)N and the GaN around it, and this mismatchincreases with In fraction x. If x gets too high or the width of thepre-well gets too large, or both, crystal defects will be formed (e.g.,dislocations) that are generally deleterious to the electron and holetransport, particularly to the electron-hole radiative recombinationefficiency. In this embodiment, there are acceptable ranges for multipleparameters across the structure—these ranges and parameters are listedin Table IV. In FIG. 14(a), layers 570 are the double barrier structure,which is shown in exploded view in FIG. 14(b). The limits on layers 571,572, and 573 are listed in Table IV. Application of the InGaN prewell tothe single barrier structure observed in FIG. 13(a) results in FIG.14(c). In this embodiment, there are acceptable ranges for multipleparameters across the structure—these ranges and parameters are alsolisted in Table IV.

TABLE IV Material Characteristics FIG. 14(a) - Double barrier UDRT-LEDwith InGaN prewell top contact region n-type doping concentration:5E16-2E20 cm⁻³ top contact region thickness: 10-500 nm top contactregion composition: GaN, InGaN (1%-30% In), AlGaN (1%-90% Al) UID topspacer thickness: 2-15 nm UID top spacer composition: GaN, InN, InGaN(1%-99% In) UID InGaN prewell thickness: 1-8 nm UID InGaN prewellcomposition: InN, InGaN (5%-99% In) bottom contact region n-type dopingconcentration: 5E16-2E20 cm⁻³ bottom contact region thickness: 10-1000nm bottom contact region composition: GaN, InGaN (1%-30% In), AlGaN(1%-90% Al) substrate: GaN, Sapphire, AlN, SiC, Si FIG. 14(b) - Doublebarrier for 14(a) UID top barrier composition: AlN, AlGaN (40%-99% Al)UID top barrier thickness: 0.5-5 nm UID quantum well composition: GaN,InN, InGaN (1%-99% In), AlGaN (1%-70% Al) UID quantum well thickness:1-8 nm UID bottom barrier composition: AlN, AlGaN (40%-99% Al) UIDbottom barrier thickness: 1-8 nm FIG. 14(c) - Single barrier UDRT-LEDwith InGaN prewell top contact region n-type doping concentration:5E16-2E20 cm⁻³ top contact region thickness: 10-500 nm top contactregion composition: GaN, InGaN (1%-30% In), AlGaN (1%-90% Al) UID topspacer thickness: 2-15 nm UID top spacer composition: GaN, InN, InGaN(1%-99% In) UID single barrier composition: AlN, AlGaN (40%-99% Al) UIDsingle barrier thickness: 0.5-5 nm UID InGaN prewell thickness: 1-8 nmUID InGaN prewell composition: InN, InGaN (5%-99% In) bottom contactregion n-type doping concentration: 5E16-2E20 cm⁻³ bottom contact regionthickness: 10-1000 nm bottom contact region composition: GaN, InGaN(1%-30% In), AlGaN (1%-90% Al) substrate: GaN, Sapphire, AlN, SiC, SiUnipolar-Doped LED with Delta-Doped “Pre-Well”

An In-composition pre-well is a promising idea but adds difficulty tothe epitaxial growth because of the lattice-mismatch induced strain, andbecause it requires a third cationic species in addition to Ga and Alduring the epitaxial growth. A simpler way to create a “pre-well” thatcan support light emission at wavelengths in the visible region belowthe GaN band gap may be delta doping. This is a common technique in MBEgrowth and quite effective when the dopant incorporates as well as Sidoes in GaN. With n-type delta doping, a triangular-like potential wellis created in the region around the delta-doped region as shown in FIG.15(a). Electrons will occupy quantum states in this well and willrealize degenerate populations in the lower of these states, much likein the InGaN quantum well.

The triangular band bending that occurs in the delta-doped regiongreatly affects the electron potential, so a similar-but-opposite effectmust occur for the holes in the valence band. In other words, the holesin the delta-doped region see a potential barrier, not a potential well.This then makes the quantum well between the AlN barriers the mostlikely region of occupancy for the holes from a thermodynamic standpointwith a corresponding hole sheet density that is very high and fed by thestrong Zener tunneling from the collector side. Although the electronsheet density in the delta-doped pre-well and the hole sheet density inthe AlN-barrier confined quantum well are spatially separated, theirmutual sheet densities should be high enough that the electron-holerecombination efficiency is sufficiently high for LD or LED operation.In this embodiment, there are acceptable ranges for multiple parametersacross the structure—these ranges and parameters are listed in Table V.In the previous embodiment, layers 350 were described as a doublebarrier. Here the double barrier is more clearly identified. The limitson layers 351, 352, and 353 are also listed in Table V. Application ofthe delta-doped prewell to the single barrier structure observed of FIG.13 results in FIG. 16. In this embodiment, there are acceptable rangesfor multiple parameters across the structure—these ranges and parametersare listed in the Table V.

TABLE V Material Characteristics FIG. 15(a) - Double barrier UDRT-LEDwith delta-doping top contact region n-type doping concentration:5E16-2E20 cm⁻³ top contact region thickness: 10-500 nm top contactregion composition: GaN, InGaN (1%-30% In), AlGaN (1%-90% Al) UID topspacer thickness: 2-15 nm UID top spacer composition: GaN, InN, InGaN(1%-99% In) UID bottom spacer thickness: 1-5 nm UID bottom spacercomposition: GaN, InGaN (1%-100% In), AlGaN (1%-90% Al) delta dopingwith an n-dopant at dose levels between 10¹² and 10¹⁴ cm⁻² bottomcontact region n-type doping concentration: 5E16-2E20 cm⁻³ bottomcontact region thickness: 10-1000 nm bottom contact region composition:GaN, InGaN (1%-30% In), AlGaN (1%-90% Al) substrate: GaN, Sapphire, AlN,SiC, Si FIG. 15(b) - Double barrier for 15(a) UID top barriercomposition: AlN, AlGaN (40%-99% Al) UID top barrier thickness: 0.5-5 nmUID quantum well composition: GaN, InN, InGaN (1%-99% In), AlGaN (1%-70%Al) UID quantum well thickness: 1-8 nm UID bottom barrier composition:AlN, AlGaN (40%-99% Al) UID bottom barrier thickness: 1-8 nm FIG.15(c) - Single barrier UDRT-LED with delta-doping top contact regionn-type doping concentration: 5E16-2E20 cm⁻³ top contact regionthickness: 10-500 nm top contact region composition: GaN, InGaN (1%-30%In), AlGaN (1%-90% Al) UID top spacer thickness: 2-15 nm UID top spacercomposition: GaN, InN, InGaN (1%-99% In) UID single barrier composition:AlN, AlGaN (40%-99% Al) UID single barrier thickness: 0.5-5 nm UIDbottom spacer thickness: 1-5 nm UID bottom spacer composition: GaN,InGaN (1%-100% In), AlGaN (1%-90% Al) delta doping with an n-dopant atdose levels between 10¹² and 10¹⁴ cm⁻² bottom contact region n-typedoping concentration: 5E16-2E20 cm⁻³ bottom contact region thickness:10-1000 nm bottom contact region composition: GaN, InGaN (1%-30% In),AlGaN (1%-90% Al) substrate: GaN, Sapphire, AlN, SiC, Si

Unipolar-Doped Interband Ultraviolet Emitter

There are applications at shorter wavelengths, such as UV sterilizationof domestic water and of exposed surfaces in hygienic settings (likesurgery rooms), which are growing in popularity. This is because certainwavelengths of UV radiation are very effective in killing dangerousbacteria like E-coli. The wavelength range that has been found to bemost effective is 240-280 nm because this is where the DNA of thebacterial cells is broken by the UV photons in an irreversible waydescribed later in this specification. The 240-280 nm range lies in themiddle-UV region (defined historically as λ=200-300 nm), but betterdefined technologically as the UV C-band (λ=100-280 nm). And thesewavelengths have been difficult to realize in a facile way usingconventional GaN-based LED/LD technology, as the p-type doping parasiticresistances are exacerbated with the wider bandgap Al-containingAlGaN-based structures to reduce the emission wavelengths.

The presence of Al in the alloy increases the direct bandgap to 4.77 eV,consistent with the fact that the GaN bandgap is ≈3.4 eV, and the AlNbandgap is ≈6.2 eV, and the ternary alloys always tend to have a bandgapintermediate between those of its binary constituents.

Otherwise, the behavior of the device is the same as that showngraphically in FIG. 3(a). Under “forward” bias (positive potentialapplied on the collector side relative to the emitter side), electronsaccumulate on the emitter side to create the “supply” for resonanttunneling. Holes are created on the collector side by Zener tunneling,and these holes then tunnel to the emitter side where they participatein radiative recombination with electrons at the bandgap photon energy(4.77 eV), which corresponds to a photon wavelength of 260 nm—right inthe middle of the UV-sterilization band. FIG. 17 shows the proposed UVemitter design. In this embodiment, there are acceptable ranges formultiple parameters across the structure—these ranges and parameters arelisted in Table VI.

TABLE VI Material Characteristics FIG. 17 - AlGaN UV Emitter top contactregion n-type doping concentration: 5E16-2E20 cm⁻³ top contact regionthickness: 10-200 nm top contact region composition: AlGaN (1%-95% Al)UID top spacer thickness: 2-8 nm UID top spacer composition: GaN, AlGaN(1%-20%Al) UID bottom spacer thickness: 1-50 nm UID bottom spacercomposition: AlGaN (1%-80% Al) bottom contact region n-type dopingconcentration: 5E16-1E20 cm⁻³ bottom contact region thickness: 10-1000nm bottom contact region composition: AlGaN (1%-95% Al) substrate: AlNor sapphire

Unipolar-Doped Resonant Tunneling Light Emitting Diode—LightingApplications

Wavelengths of light in the visible window (˜390-700 nm) have manyapplications, however, despite good efficacy, high up-front costs,reliability concerns, and compatibility issues have deterred manyconsumers. Much of this cost is incurred due to complex manufacturingtechniques which are needed to navigate the p-type dopant related issueswhich were discussed earlier. Many of these issues can be eliminated byusing the devices according to the innovation.

One common method for achieving white light is to use high energynear-UV/blue emitters to pump phosphors which down-convert to green andred, thus allowing for white light production when mixed. Another methodis to simply use multiple primary (red, green, blue, amber) LEDs,however this introduces complexity because each must be grownseparately. With no phosphor down conversion losses, however, this couldresult in very high efficiencies. Using the devices according to theinnovation should directly enable either method. The first method wouldbe possible by using the designs in FIGS. 10-17 and selecting thevarious parameters in Tables I, II, III, IV, V, and VI in such a way asto allow for near-UV/blue (˜350-450 nm) emission which would be utilizedto pump the appropriate phosphors and down-convert to green and redlight emission, thus allowing for white light when mixed with the blue.The second method would be possible by using the designs in FIGS. 10-16and selecting the various parameters in Tables I, II, III, IV and V insuch a way that would allow for light emission at the required colors(red, green, blue, amber, etc.).

Another possibility would be to use the innovations detailed previously(FIGS. 10, 12, and 14) as a replacement for the p-type layer in priorart white LEDs. State-of-the-art LEDs are pn junctions, wherein thep-type layer is responsible for injecting holes into the active region,thus allowing for recombination with the injected electrons from then-type layer. Then, application of one of the two methods described inthe previous paragraph for creating white light is applied. Using thedevices according to the innovation, should allow for the completereplacement of the p-type layer. This would be possible because thestructures detailed previously (FIGS. 10, 12, and 14) could be used as ahole injection source, thus making the device completely n-type.

GaN Unipolar-Doped Interband Vertical Cavity Laser and LED

Combining the proposed UDRT light emitters with a GaN/AlN dielectricmirror (distributed Bragg reflector), may provide the unipolar n-dopedVCSEL structure shown in cross section in FIG. 19. Lacking the p+ dopedregions of the traditional GaN p-n VCSEL of FIG. 18, the unipolar-dopedGaN heterostructure can support higher currents without a lossy ITOlayer. In fact, the VCSEL structure of FIG. 19 is very much like thehighly successful GaAs-based p-n diode VCSELs in widespread use aroundthe world today. But the unipolar-doped GaN structure offers advantagesover the GaAs structure, including superior thermal characteristics.Both GaN and AlN have much higher thermal conductivity than theircounterparts GaAs and AlAs, which allows the VCSELs to be driven withmore current, and emit more power, before thermally induced failure.

Even in the absence of lasing action, perhaps because of optical lossesin the n++ layers, the structure in FIG. 19 will still operate as a“super” LED because the dielectric mirrors will favor vertical emissionover lateral emission. This will also reduce the number of layers of GaNand AlN required in the dielectric-mirror stack. Deposited dielectricmirrors that lack critical thickness constraints, could also beutilized. The top would be as-grown and the bottom could be added aftersubstrate wafer thinning.

The LED and LD light emission according to the innovation rely onelectron-hole recombination. The holes are generated by a localinterband tunneling process that occurs because of the strong electricfield created by the polarization and piezoelectric effects. This isquite different from p-n junction GaN LED/LDs where the holes necessaryfor cross-gap emission are created by classical drift from the p-dopedregion into the depletion region, as shown in FIG. 1. Essentially, holesare generated around the very physical location they are needed, withoutthe troublesome issues of bringing them in from the anode contact.

The unipolar n-doped VCSEL structure can be primarily designed for 360nm lasing, but can also be tailored for long wavelength lasing as wellif one of the prewell or MQW structures are implemented (Tables II, III,IV, and V). Furthermore, UV lasing (240-360 nm) can be accomplished byimplementing the VCSEL design with the embodiment detailed in Table VI.

Optoelectronic Circuit Application GaN RTD-LED/LD Optical Clock

Through the combination of NDR already demonstrated, and intense lightemission, new device concepts that combine both fascinatingcharacteristics can be envisioned. The first is an optical clock. It iswell known that the NDR region of RTDs can support electricaloscillations of two different types. The best known is a sinusoidaloscillation created by connecting the RTD to a lumped-element LC tankcircuit. By biasing the RTD into the middle of the NDR region, andassuming the tank circuit has low enough losses, the resultingsinusoidal oscillation frequency is f≈[2π(LC)^(1/2)]⁻¹. But it requireslow-loss inductors and capacitors, which are non-trivial to fabricatemonolithically on the same substrate as the GaN RTD/light-emitter ofFIG. 20.

A better approach to make RTDs oscillate is the transmission-linerelaxation oscillator approach of FIG. 20(a) Here the RTD is connectedto a transmission line that is shorted at one end, the transmission lineshown in FIG. 20(a) being a coplanar waveguide. The RTD is then biasedto just below the peak voltage or just beyond the valley voltage of FIG.20(b), and induced to switch to the valley or peak points, respectively,by a small variation or fluctuation of the bias voltage. The fastswitching action of RTDs creates a voltage pulse that propagates downthe transmission line to the short, and then reflected and inverted insign. When the reflected pulse gets back to the RTD, it induces thealternate switching event. Given a low-loss transmission line and weakcoupling through the DC bias circuit, the sequence of pulses isself-sustaining as shown graphically in FIG. 20(c), and has repetitionfrequency f_(rep)≈(4Tp)⁻¹ where T_(P) is the pulse propagation timebetween the RTD and the short circuit.

Now suppose that the RTD structure is the GaN unipolar-dopedlight-emitter, and that the light emitting threshold voltage is close tothe peak voltage of the NDR region as shown graphically in FIG. 21(a)(violet curve). Then, as the RTD switches between the peak and valleypoints during sustained relaxation oscillations, the light emitted bythe GaN structure is varying from off-to-on with a difference thatdepends on the width of the NDR region in voltage. But independent ofthis difference, the emitted light should have a square-wave likebehavior in the time domain, as shown in FIG. 21(b) (violet curve). Inother words, it should behave as an optical “clock”. The rise and falltimes of the light waveform has not been characterized, but judging fromthe unipolar nature of the GaN emitter, could be much faster than therise and fall times of a conventional p-n LED emitter.

The optical clock according to the innovation provide frequencystability, usually quantified by timing “jitter” of the rising andfalling edges. RTD relaxation oscillations are known to be extremelystable, in part because the peak and valley points of unipolar RTDs arerobust with respect to temperature, power supply fluctuations, etc.Hence, it is expected that the resulting optical clock will also be verystable. And unlike alternative optical clocks, such as atomic clocks,the RTD-emitter optical clock can be integrated into GaN circuits tocarry out optical signal processing, lidar transmission, and perhapsoptical computing.

System Application: Ultraviolet Sterilization Background

As mentioned above, a useful application of the UV emission from theunipolar-doped GaN tunneling structures is sterilization of domestic(e.g., drinking) water. Water cleanliness is still a major healthproblem worldwide because of the many harmful bacteria that act aspathogens. This includes enterotoxigenic Escherichia coli, or E. colifor short (a primary cause of chronic diarrhea, and 7^(th) leading ofdeath worldwide); Vibrio cholerae (the primary cause of cholera); and avariety of Campylobacter species (primary cause of campylobacteriosis).A related application is sterilization of food and hygienic surfaces,including those in clinical and industrial laboratories. This alsobenefits from the generic vulnerability of bacteria to UV light, a goodexample being Salmonella enterica (serotype Typhi; a primary cause oftyphoid fever); 40 species of genus Staphylococcus (a common cause ofbacterial infections in open wounds or from surgery); and approximately50 species of genus Streptococcus associated with a variety of maladiesincluding bacterial pneumonia and necrotizing fasciitis (i.e., “flesheating” bacteria). Protozoa and viruses are two other types of pathogenstargeted by the technology. For example, most of the waterborneprotozoan specimens like Giardia lamblia will succumb to powerful UV inthe same 240-280 nm band. And most if not all of the influenza virusstrains that scourge the world every year are subject to sterilizationby the same UV wavelength region. However, protozoa are generallywaterborne, and influenza viruses are either airborne orsurface-bearing. As described herein, different optical-couplingmethodologies must be adopted to deal with these diverse organisms.

Independent of the specific organism, the UV-C region between λ˜240 and280 nm (the sterilization band) is particularly advantageous, anddisplays a typical “germicidal effectiveness” curve as shown in FIG.22(a) In this range the UV photons break the inter-atomic bonds in DNAand RNA molecules of all sorts, not just those of bacteria.Interestingly, in DNA the damage starts by breaking the bond between thenucelobase thymine and its complementary partner adenine, and then thepermanent damage occurs when the liberated thymine molecule re-bindswith another UV-liberated thymine if it happens to be a nearest neighborin the DNA molecule. The resulting thymine “dimer” is stablethermodynamically so doesn't get repaired by normal biologicalprocesses. An individual cell or virus with its nuclear DNA so inflictedis incapable of reproducing properly; hence, it becomes sterile. Atsufficiently high doses, all waterborne enteric pathogens are sterilizedby UV radiation. The general order of microbial resistance (from leastto most) and corresponding UV doses for extensive (>99.9%) sterilizationare: (1) vegetative bacteria and the protozoan parasites at low doses(1-10 mJ/cm²), and (2) enteric viruses and bacterial spores at highdoses (30-150 mJ/cm²).

The sterilization band is superimposed on the transmittance behavior ofclear water in FIG. 22(b) where a low absorption coefficient (<1 m⁻¹) isseen. This means even after 1 m of propagation, the majority of UV-Cphotons remain unabsorbed. So if bacteria are the only other absorbingconstituent, it would have to be in a relatively high concentration tochange the behavior in FIG. 22(b). But in the type of water generallytreated by UV for sterilization, the concentration of bacteria isgenerally low in an absolute sense. Hence an important metric becomesthe external killing efficiency (EKE), which is defined by the ratio ofthe number of bacterial cells sterilized to the number of UV-C photonsincident on the sample.

Existing Technology

The established technology for water sterilization is eitherlow-pressure mercury discharge tubes or solid-state LEDs. Hg dischargetubes are similar to a normal fluorescent light bulk but containing aninert gas plus a few drops of liquid mercury which ionizes and forms aplasma once electrical bias is applied. Ionized mercury atoms have astrong emission centered at λ=254 nm—in the middle of the sterilizationband. Most low-pressure mercury lamp UV disinfection systems can readilyachieve UV radiation doses of 50-150 mJ/cm² in high quality water, andtherefore efficiently disinfect essentially all waterborne pathogens.However, like most “discharge” tubes, the mercury bulbs degrade inperformance gradually over time and so must be replaced periodically,generally at least once per year. In addition, they require wall socketvoltages (110 V or above) to operate properly, which in remote locationsmay or may not be available, Finally, they are subject to severalfailure mechanisms, perhaps the most damaging being the breakage of thebulb and the transfer of the highly toxic liquid mercury into the waterbeing sterilized.

Because of these drawbacks, LEDs have been pursued for watersterilization applications for decades, the first promisingsemiconductor material being SiC. However, as an indirect bandgapsemiconductor, SiC is not nearly as efficient in emission as GaN andcannot practically be designed for the sterilization band 240-280 nm.Hence, GaN has been pursued for the past 10 years or so but it too hasproblems emitting in the sterilization band since the room-temperaturebandgap of GaN corresponds to a wavelength of 360 nm. And as describedin detail above, GaN brings significant challenges in p-type doping,ohmic contacts, etc., and each challenge brings with it significantcost. For example, a fully packaged GaN LED operating around 250 nmcosts ˜$600 for a 1 mW light emission.

Improved UV-C Unipolar-Doped GaN LEDs and Two-Dimensional Arrays

It is possible to fabricate 2D arrays of the GaN/AlN LEDs takingadvantage of the simplified growth and fabrication, and uniformity inperformance that the unipolar-doping approach provides. This shouldcontinue to be true even for the UV-C compatible LED design having AlGaN(Al fraction≈0.5) instead of GaN in the cladding layers outside thetunneling region. However, to be superior to existing LED technologybased on SiC- or GaN-based emitters, there needs to be an improvement inthe external quantum efficiency (EQE) as measured by the ratio of thetotal rate of emitted UV-C photons that escape the device structure tothe electron current crossing the device structure. Equally important isto ensure that this EQE is dominated by vertical emission since that isthe emission direction that reinforces as multiple devices are combinedto form 2D arrays. Since the UV emission within the active region of theunipolar-doped LED is approximately isotropic, the EQE problem ischallenging and multi-faceted.

There are two concepts for improving the EQE greatly in the verticaldirection. The first, shown in FIG. 23(a) entails a top ohmic contactthat is transparent to the UV-C radiation between 240 and 280 nm in thesterilization band. Transparent ohmic contacts (TOCs) are not new in thephotonic field, having been used for decades on detectors and emittersalike. A good example is indium-tin-oxide (ITO)—a semi-metallic alloythat offers good electrical conductivity at low frequencies (down toDC), but very low conductivity at infrared wavelengths. In other words,ITO acts like a metal at or near DC but an insulator at IR or shorterwavelengths. The physics behind this remarkable behavior is theplasma-edge effect in optics, whereby the material strongly reflectsradiation at frequencies below the plasma resonance, but transmitsradiation above the resonance. However, if the material is crystallineor polycrystalline, it is also important that its “effective bandgap”occur at shorter wavelengths than 260 nm, or equivalently at energygreater than ˜4.6 eV. This then requires a wide bandgap material withhigh electrical conductivity, but not too high. Two candidate materialsare Ga₂O₃, and BN.

There is no reason why the same effect cannot be used to make a metalalloy act like a good conductor at or near DC, but be transparent in thesterilization band. The important point here is that the metal must bekept very thin, approximately the classical skin depth or less. Onepromising candidate will be a Ti:Ni:Au alloy. The Ti in such an alloywill provide for good adhesion of the metal to the GaN on top of theactive device mesa, as shown in FIG. 19. However, consistent with metalalloy physics, the Ti:Ni:Au alloy will have optical conductivity lessthan either of its three constituents, so probably display a plasma edgein the visible or near-UV regions. Note that this TOC approach isfacilitated by the relatively short wavelength 240-280 nm of thesterilization band compared to any of the visible bands (violet, blue,or green) commonly being pursued in GaN-based LEDs for lightingapplications. In general, the shorter the wavelength, the easier it isto find a metal displaying a plasma-edge at a longer wavelength.However, getting the effective bandgap(s) to wavelengths below the240-280 nm band is another matter.

The second concept for improving the vertical EQE is displayed in FIG.23(b). It consists of an embedded GaN quantum-dot layer created duringepitaxial growth and located at some distance below the active region ofthe GaN-based LED. Its role is to reflect UV-C radiation that is emitteddownward by the active device, back toward the vertical direction andtransmitted through the TOC. This takes advantage of the fact that toemit in the sterilizing radiation band (240-280 nm), Al_(x)Ga_(1-x)N(x≈0.5) must be used in the cladding layers on both sides of the activeregion. But at such high Al fraction, it is possible to create GaNquantum dots spontaneously during the epitaxial growth. In addition,because the GaN bandgap is smaller than the Al_(x)Ga_(1-x)N, the GaNquantum dots will be optically active (i.e., be occupied by electrons),meaning that their bound transitions will be dipolar and will scatterradiation very effectively. The obvious beneficial aspect of suchresonant scattering is that it produces strong back-reflection. In otherwords, the embedded layer of GaN quantum dots in FIG. 23(b) will actlike a “distributed mirror”. UV-C photons emitted in the active regionand downward-directed will be reflected by the “distributed mirror”,pass back through the active region (with a small probability ofabsorption), and then transmit through the top TOC and contributepositively to the EQE performance metric.

Given these improvements in the EQE through LED materials design andfabrication, the extension to 2D arrays becomes relativelystraightforward. FIG. 24 shows such an array as a collection of LED“stripes”. Such stripes are known to be electronically and thermallystable compared to large-area device alternatives, so a 2D array of themshould work well provided their inter-strip separation is great enough.Assuming each stripe has the vertical EQE technology shown in FIGS.23(a) and (b), the power emitted vertically should increase linearlywith the number of stripes in the array.

Optical Coupling for Sterilization Applications

Given the above conceptual improvements in the unipolar-doped GaN LEDstructure and its extension to 2D emitting arrays, there remains theissue of coupling the resulting 2D UV-C arrays into free space for theanticipated biological sterilization applications. And this depends onthe two targeted applications: (1) sterilization of domestic drinkingwater, and (2) sterilization of hygienic surfaces in locations likehospital surgery rooms.

FIG. 25(a) shows the anticipated application of the 2D unipolar-dopedGaN LED arrays to the sterilization of water. It is based on fourplausible assumptions: (1) that the water to be sterilized will beflowing through a round tube (or “pipe”); (2) that the tube diameter isless than the ˜1 m absorption length for UV-C photons in clear waterdisplayed in FIG. 22(b); (3) that the tube can be coated on the innersurface with a metal that reflects photons in the sterilization bandvery effectively, and (4) the water is “clear”, free of turbidity(likely from a previous filtering step), and contains but a smallconcentration of toxic cells or other bioparticles. For assumption#3, agood candidate metal would be titanium or one of its alloys which areboth chemically inert and have a plasma-edge wavelength well below thesterilization band. Given these assumptions, then the powerfulunipolar-doped GaN LED array can be located external to the water tubeand coupled into the tube through a quartz window. The vast majority ofthe sterilizing UV photons will then enter the tube but not be absorbedon the first pass because of low turbidity and bacterial concentration.However, the metal coating on the inner surface of the sterilizing tubesection will reflect the unabsorbed UV-C photons, giving them a secondchance for absorption by the bacteria or other biomaterial in the tube.Moreover, if they are not absorbed on the second pass, then a subsequentreflection will occur, giving them a chance for absorption on the thirdpass, etc. The so-called “re-cycling” of UV photons will create a muchhigher external kill efficiency (EKE) than would otherwise occur. Andthis means that the UV-C LED array can have much lower power (and thuslower cost) than if the photons had only a single pass through thewater.

FIG. 25(b) shows the anticipated application of the 2D unipolar-dopedGaN LED arrays to the sterilization of hygienic surfaces. The concepthere is essentially a hand-held UV-C “pistol” with the GaN LED arraylocated at the focal point of a garden-variety parabolic reflector muchlike those used in spot lights and in the headlights of automobiles.This results in a UV-C collimated beam that can be directed under humanoperation onto any surface or object of choice, be it the flat surfacesin a hospital surgery room, or the door knobs and handles in a buildingor facility subject to an “outbreak”. It is remarkable that at thepresent time, well into the 21^(st) century, many live in fear of thenext flu epidemic knowing full well that it can be easily caught byopening a door or ingesting the airflow in a commons area. The disclosedUV-pistol instrument could change that predicament dramatically.

What is claimed is:
 1. A solid-state device, comprising: a bottom n-typelayer; a top n-type layer; a middle layer inserted between the top layerand bottom layer, where the middle layer comprises at least twomaterials provided between the top and bottom layers which serve asheterojunction tunnel barriers; and where the top layer and the middlelayer form an interband tunnel barrier to generate holes by Zenertunneling across the potential barrier of the forbidden energy gap, andwhere the middle layer forms at least one intraband tunnel barrier tocontrol electron flow.
 2. The device of claim 1, wherein the top, middleand bottom layers are comprised of gallium nitride, aluminum nitride,indium nitride or alloys and combinations of III-nitride semiconductorsor III-nitride compatible semiconductors.
 3. The device of claim 2,wherein the heterojunction interband tunnel barrier is formed by thepolarization effects at III-nitride heterojunctions.
 4. The device ofclaim 1, wherein the middle layer forms at least two intraband tunnelbarriers, wherein the at least two intraband tunnel barriers form aquantum well within the middle layer.
 5. The device of claim 1, whereinthe middle layer forms at least two intraband tunnel barriers, whereinthe at least two intraband tunnel barriers form a double barrierresonant tunneling diode.
 6. The device of claim 1, wherein the middlelayer is either undoped or doped less than the top and bottom n-typelayers.
 7. The device of claim 6, wherein the middle layer also includesa spacer layer between the first barrier and the bottom n-type layer,the middle layer and the bottom n-type layer, or both.
 8. A lightemitting diode, comprising: a bottom n-type layer; a top n-type layer; amiddle layer inserted between the top layer and the bottom layer, wherethe middle layer comprises at least two materials provided between thetop and bottom layers which serve as heterojunction tunnel barriers; andwhere the top layer and the middle layer form an interband tunnelbarrier to generate holes by Zener tunneling across the potentialbarrier of the forbidden energy gap, and where the middle layer forms aleast one intraband tunnel barrier to control electron flow; and wherethe radiative recombination of Zener injected holes from the top layeroccurs directly with electrons electrically injected from the bottomlayer.
 9. The light emitting diode of claim 8, where p-type doping isnot part of the active device.
 10. The light emitting diode of claim 8,wherein the top, middle and bottom layers are comprised of galliumnitride, aluminum nitride, indium nitride or alloys and combinations ofIII-nitride semiconductors or III-nitride compatible semiconductors. 11.The light emitting diode of claim 10, wherein the heterojunctioninterband tunnel barrier is formed by the polarization effects atIII-nitride heterojunctions.
 12. The light emitting diode of claim 8,wherein the middle layer forms at least two intraband tunnel barriersthat form a quantum well or a double barrier resonant tunneling diodewithin the middle layer.
 13. The light emitting diode of claim 8,wherein the middle layer is either undoped or doped less than the topand bottom n-type layers.
 14. The light emitting diode of claim 13,wherein the middle layer also includes a spacer layer between the firstbarrier and the bottom n-type layer, the middle layer and the bottomn-type layer, or both.
 15. The light emitting diode of claim 14, where aregion between the spacer layer and the bottom n-type layer comprises atleast one quantum well.
 16. The light emitting diode of claim 15, wherethe barriers that form the at least one quantum well are comprised ofundoped or n-type doped gallium nitride or aluminum gallium nitridealloys.
 17. The light emitting diode of claim 15, where the wells thatform the quantum well or quantum wells are comprised of undoped orn-type doped indium nitride, gallium nitride, aluminum gallium nitrideor their alloys.
 18. The light emitting diode of claim 8, where themiddle layer includes an n-type delta-doped layer.
 19. The lightemitting diode of claim 8 where a portion of the radiatively emittingphotons are converted to longer wavelengths by the inclusion of externalphosphorescent downconverters.
 20. A laser diode, comprising: a bottomn-type layer; a top n-type layer; a middle layer inserted between thetop layer and bottom layer, where the middle layer comprises at leasttwo materials provided between the top and bottom layers which serve asheterojunction tunnel barriers; and wherein the top layer and the middlelayer form an interband tunnel barrier to generate holes by Zenertunneling across the potential barrier of the forbidden energy gap, andwherein the middle layers form at least one intraband tunnel barrier tocontrol electron flow; and wherein the radiative recombination of Zenerinjected holes from the top layer occurs directly with electronselectrically injected from the bottom layer; and wherein a Fabry-Perotetalon is added external to the radiative recombination zone to form alaser diode.